The most immersive multiplayer online RPG I have ever played, is certain servers from Neverwinter Nights (original not 2). Escape from Uncertainty is my top pick.
I have been meaning to make a thread about this server for a while, but havn't gotten around to it yet. I have been playing on this server off and on for since 2006.
Basically, it is set in an alternate telling of the Forgotten Realms, this chapter is located in the Underdark. This gives the setting a very grimdark feel and its very survivalish. Death penalties are harsh and outside of town, theres nasty things that will smack you down if you dont prepare and think on your feet. That said, if you act sensibly and play with care, you can totally explore. Just try going with friends when you first start. I often walk around solo in the wilds, but my character is well prepared and I know the ropes.
So basically, you have a DM team that breathes life into the world. The possess NPCs and craft plots, spice things up. They help you carry out your plans...
Roll play is enforced and you must stay in character at all times. The level of roleplay here is superlative but don't let that intimdate you if your new to RP, people are helpful and friendly. You can always send an out of character tell to another player or DM if you need help.
This is a conflict based setting, PVP is semi-open. There are areas where PVP is only allowed with a DM present, but lots of areas where PVP is open. Death in PVP is permanent. Players are encourage to proactively pursue their goals, be it PvE or PvP. The DMs will support this kind of play.
To wrap up, the experience lands somewhere between playing an MMO like WoW and a Role Play intensive MUD. At it's base, NWN is an actiony sort of game and I like that a lot about it. Relo playing is different here because instead of emoting things like walking across the room and picking up the mug form the table, you just do that. In general emoting is much more minimal and chat tends to be what character are actually saying, with their actions in-world reflecting all the other stuff. There is still emoting for some things.
The server is properly played with the use of the forums. While there is an in-game messaging systems, a majority of messages and posting happen there.
Worth a looksie in my opinion. Tell them Calavera sent you. Message me on this forum if you plan to go in-game, my characters name is Worm.
http://www.efupw.com/It will take a bit of setup to get going. The base game is cheap on GoG and Steam. You will need all the expansions so get the Diamond Edition. You will need the patches. I recommend using the patcher made for this server, it works great.
See here:
http://www.efupw.com/getting-started/